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Why have cargo hybrid airships not succeeded in the market?
Using a Lighter Than Air (LTA) Hybrid Airship to efficiently transport cargo to remote areas seems like an idea that should flourish.  It allows access to remote places in the world that have no airports, harbors, or roads leading to them by providing a point-to-point delivery system with vertical take-off and landing, all while providing a greener solution.  I see at least three players all trying to enter this market which seems perpetually stuck as coming about in the next 4-5 years and all seem to have benefited from the U.S. Army's Long Endurance Multi-intelligence Vehicle (LEMV) project that was canceled in January 2013.

Hybrid Air Vehicles in the U.K. has partnered with Northrop Grumman owns a prototype that came out of the LEMV project and plans to build a 50-ton capacity airship in about five years.
Aeroscraft in the U.S. has a prototype and plans to build an airship that carries a 66-ton load as shown on the History channel.
Lockheed Martin has a P-791 / SkyTug prototype derived from the LEMV program and hopes to build a SkyFreighter to lift a 70-ton cargo.

It appears none of these companies has any current orders and are all looking for funding to build their proposed cargo airships.  Apparently an investment of on the order of 50 to 250 million USD would allow one of these to be built for commercial use.  While this is a lot of money, it is small compared to other aviation investments. For example, the Boeing 787 Dreamliner required 32 billion USD as of 2011.
Today, all such airships use noble gas helium instead of the very flammable hydrogen used in the Hindenburg accident, so the historical fear of using such ships should not apply to the new technologies.  These proposed ships are planned to travel at between 70 and 115 MPH, so while not as fast as airplanes, they do compete with trucking and shipping speeds.
So the question is, given the relatively small cost to develop one of these cargo lift systems, why hasn't a company like Boeing with deep pockets wanted to invest in such systems and these companies continue to search for funding?  Is there some kind of flaw in the proposal that such a new cargo shipping approach would save money?  Is it that the market is too small to interest big companies? The hangar space required to store and maintain these ships is huge and little such infrastructure currently exists, perhaps that is their downfall?  Alternatively, maybe their maintenance requirements are their Achilles heel?

lighter than air ships are slow and plagued by the hindenburg disaster

@ratchetfreak I edited the question to add their use of helium and projected travel speeds.

How much demand is there really for cargo transport to areas without harbors, airports, or roads? Obviously we have very limited ability to supply cargo to such areas right now, which effectively limits our development in such areas, keeping the demand relatively low. While availability of an inexpensive way of delivering to such areas could spur development and thus demand that is only speculative future demand, which does little to spur initial sales. I suspect that combined with limited airship infrastructure may be the bulk of the answer.

With todays technology, hydrogen is an option for cargo - just use drone technology to pilot it. You will lose the occasional airship, but that is just business in the end.

@KevinCathcart I can envision some market for offshore and mining/oilfields in remote regions, maybe disaster relief, but the market is probably too small to make initial development economically feasible (and of course in disaster relief, you're pretty much reliant on government funding from donor countries, and they're more likely to push existing resources into the very occasional use, even if less than optimal for the purpose).

Helium becomes very expensive nowadays.

The issue with large cargo delivery to remote locations is that there is almost no demand: few people live in such places.

First of all, there is the question of demand. There aren't enough cases where a very heavy load needs to be moved to a remote location by air. There are probably even fewer where there are funds to actually pay for this option. The An-225 is currently the largest aircraft in operation, and it was only constructed under special circumstances. Although has certainly found use, they still haven't even found the funds to finish a second one which is partially built.
When this option is needed today, helicopters are deployed on cargo aircraft to the area. The helicopters may have to make multiple trips but can do the job. So the subset of cases is even smaller; where heavy cargo needs to be delivered to a remote area and also cannot be split into shipments that helicopters can handle. Deploying a large airship in this manner is also much harder. Either the airship would have to fly itself there, which could take a long time, or it would have to be reduced to a transportable size.
The airship would also need to be protected from the weather somehow. This could require either a very large (and expensive) hangar. Without a hangar, the airship must be tethered, and restraining such a large surface area in the wind would be difficult.
Wind would also affect operation of the airship. Helicopters are of course limited by weather conditions, but wind would have a much greater effect on a large and slow airship.
If the airship could be fairly easily collapsible, it would solve the issues of storage and transportation. However, this is not a simple problem to solve. This would require a large development budget. If you are comparing this to the cost of the 787 development, where are you going to find buyers for 1000 of these airships in order to break even and make them affordable?
The issue is that there needs to be a market for this kind of transportation, which currently competes at least partially with existing helicopters. Why invest in a new type of vehicle when the current option works fine for most cases?

Helium is not without its own issues. While it isn't a fire hazard it is a limited resource. Let Popular Mechanics tell you about supply issues for helium. So any large scale industrial use of helium could drive up the price of the gas.

Not prohibitively for a small number of airships. But any large scale use would make it even more expensive.

https://www.scientificamerican.com/article/new-extremely-valuable-helium-deposit-discovered-in-africa/

There are two primary difficulties with lighter-than-air cargo craft that none of the named companies address; and which give pause to companies such as Boeing.
First: the continued dependence upon very large hangars; both for construction of large airships, and for their protection and maintenance.   Any viable cargo fleet would comprise a fair number of craft, and an added investment into numerous huge buildings.  This is an obvious "no win" situation.
Second: Virtually all airships continue to use flexible, lightweight fabrics or laminated materials for their hulls.   In order for the envelopes to be gastight, weatherproof, resistant to degradation in ultraviolet light, and still have the tensile strength needed, it becomes too expensive to create large fleets of airships.  Indeed, the materials used for modern airships are similar to those used for space suits; and there is an extremely limited number of suppliers or sources for these types of exotic and high tech materials.
Happily, there is a solution!
First, design airships that no longer need hangars.  Not for construction, and not for storage or protection.
Second, use commonly available materials such as aluminum and carbon fiber to build airship hulls.
who is doing this?
Me.

Maybe some links to your work and research would be a little better than just a self plug?  Just to fit in with the flavor of the site a bit better.  Plus, a couple links to back up an answer are always welcome :).

I'm a bit concerned that turtleairships.com says "Database Error" - plus this skeptical report and 2007 reports of a planned 2009 round the world flight that I guess didn't happen.

Raw material and suppliers are too expensive and too few for large scale production... Until we ramp up to large scale production.

An airship can be used for multiple purposes:

As a cargo airship. I had put up a proposal to Government of India for transporting goods within country using Hybrid Airship.

Can transport goods in remote location where it is difficult and
expansive to build airport but cost of building maintenance in
nearest major city will is not an issue.
Currently, there is huge pressure to save environment, and India
has pledge to reduce gases at recent summit.
Cost of transport will be quicker than by road.

Since Hybrid Airship can remain for couple of days. Can use to fit military hardware for early warning system. Need to monitor border areas especially in north west and north east.
can be also used for tourism department.
Aerial survey during floods. Every year there are floods in northern part of India. It can monitor and take necessary action.

Airship is multi-purpose vehicle, even though its speed is less compared to other mode. It was used to build dome in UK by delivering huge steel panels for building dome.
As regards to the cost, it can mooted out to reach break-even point. For developing Government and Private partnership along with other players (more number of players, lesser the burden) in such projects can help to initiate commercial use of airship.
Reasons are: (i) It is not feasible cost, which is very high; (ii) fear of accident. There was fatal accident when Boeing 747 was carrying lithium batteries from Dubai, UAE airport. Yet to get confidence to operate; (iii) Unlike other commercial aircraft, frequency is not available to use. Market still needs confidence to use, except few instances of use by tourism sector. Government takes initiative, it might help to develop market.

This is definitely relevant, but you haven't answered the original question of why such airships haven't succeeded in the market so far.

Reasons are: (i) It is not feasible cost, which is very high; (ii) fear of accident. There was fatal accident when Boeing 747 was carrying lithium batteries from Dubai, UAE airport. Yet to get confidence to operate; (iii) Unlike other commercial aircraft, frequency is not available to use. Market still needs confidence to use, except few instances of use by tourism sector. Government takes initiative, it might help to develop market.

I suggest you add those comments to your answer, it would help to make it more complete and directly answer the question.

Large airships are vulnerable to bad weather, specifically thunderstorms. Their very high surface area means a thunderstorm can easily subject them to loads in excess of their design capacity, while their relatively slow speed makes avoiding storms difficult.
The US Navy operated two large airships in the 1930's, the Akron and Macon, complete with fighter aircraft onboard. Both were lost to storms, as was the British R101 (although design deficiencies were a major contribution to that crash).

Most likely, the reason is speed and payload capacity.  Blimps and airships are really slow - around 40-50 knots which doesn’t make them competitive with large freight aircraft capable of approx 500 knots.  They are also quite large and not really compatible with existing Aviation infrastructure like hangars, etc.  given that, I’m not surprised they haven’t generated a whole lot of buyers.

They don't have to compete on speed if the price is right.

True.  However I doubt they could compete with other slower means like cargo ships or rail cars.

Well actually hybrid airships kind of defeat the main advantage of airships (Does not require fuel to have lift) While using surface area for lift occasionally isn't a bad idea, when 40 percent of the lift is in surface area and 60 percent is the helium it makes it slow and inefficient and it still requires a runway. Lockheed's largest airship has a top speed of 60, while the Hindenburg, which was created using 90 year old technology, had a top speed of 80.
but it is actually be more cost effective to send 8 airships across the pacific multiple times carrying cargo than it would be to send a single enormous, slow innefficient cargo ship across the Atlantic. I did the math a while ago so I do t remember the numbers, but it could be something like 2.3 million dollars more cost effective and take about one month less time to transfer the same amount of cargo a regular sized cargo ship against 8 cargo airships with 200 tons carrying capacity flying at 150 mph. This isn't taking airship manufacturing Vs. cargo ship manufacturing into account, this is simply fuel based. This doesn't count personell either.     If I were to take manufacturing into account, It would be even more in favor of the airships because it is quicker to produce a hydrogen fueled airship and costs a WHOOOOOOLE lot less.

It's not totally clear whether you are saying airships or cargo ships is cheaper. Also, it would help to provide some more information about where your numbers came from.


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Cessna 172 steering system and torque link purpose
How does the Cessna 172 steer while taxiing? Is it by using differential braking (braking one wheel at a time)?
Are there any connections between rudder or toe brake and nose landing gear related to steering?
What is the purpose of torque link in front of the oleo strut?
Does the nose wheel even turn?

Picture will be added tommorow , couldn't find in Google.

Please ask the question regarding the 421 fairing in a separate question.

The Cessna 172 (152, 175, 177, and 182 as well as others) all use steerable nose wheel by pushing (not depressing) the rudder pedals in the direction you want to go. If you want the nose to turn to the right, you push the right pedal forward, same with the left. You can augment this with differential steering if tighter turns are needed.
So there are actually 3 ways to steer the 172:

The nosewheel steering using rudder pedals. This puts pressure on the bungee system that turns the nose wheel left/right. Pushing the pedal doesn't directly move the wheel, you need to move the aircraft forward a little bit for the wheel to turn.
Differential braking can help make tight turns, this causes the nose wheel to "caster" or turn based on which direction you are attempting to turn the aircraft.
Some turning force is provided by the tail rudder, it isn't usually a lot but it is noticeable when landing.

Is there any connections between rudder or toe brake and nose landing gear related to steering? What is the purpose of torque link in front oleo ?

There is no connection between the rudder or toe brake and the wheel (in the purest sense, the rudder pedals are also connected to the bungees for steering), the rudder pedals  move the bungee system that puts pressure on one side or the other of the torque link on the oleo strut.

Does nose wheel even turn ?

Yes! All tricycle gear aircraft with a nose wheel must allow the nose wheel to turn at least a little bit. The 172 allows the rudder pedals to give up to 10° of wheel movement while differential braking will make the wheel move up to 30°, this is outlined in Section 7 of the 172 POH (I won't link the entire POH here, but this is the excerpt):

Source: 172SP Pilots Operating Handbook

If you update your question with a picture of the fairing you are talking about I can comment on that, but really it should be a separate question.

A notable exception is the Cessna 162 (SkyCatcher), which has a free-castering nose wheel, not a steerable one.   Its steering is done entirely with differential braking (and full rudder, but the rudder has very little effect at taxi speeds)

@abelenky Yeah, thats why I skipped the 162 (that and almost everybody forgets about that one, it isn't very popular). The Cirrus SR20/22 are also caster nose-wheel aircraft. There are also tail dragger versions of the 172 (170) and the 152 (150).

Good Morning Knight, I'm a Cessna 172S, 172N,172R pilot with over 60 hours on type. When turning a 172 I use the rudder peddles with the toe breaks to turn. The nose wheel does turn as if you look at a aircraft nose wheel no straight you see the rudder is slightly out. I would suggest having a chat to a Flight instructor to be able to look more in-depth with your questions and being able to see it practical

Turning with toe brakes is good for sharper turns in the 172. Turning with just rudder pedal should also work to steer the nosewheel; if not, the plane should be looked at.   The rudder pedals are connected to the nosewheel thru a bungee system for 10 degrees of turning. Using the toe brakes the steering turn may be increased to 30 degrees.    See the Ground Control section in Section 7 of the 172 POH.

they don't like turning on the ground so you will need to add a bit of toe break when turning to get around a corner effectually

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Explain how the infrared flame detector work?
For fire detection systems, such as for engine fire detection, how does the infrared flame detector work?

Welcome to aviation.SE! Can you explain what exactly you mean by "infrared flame detector"? Do you mean an IR system for tracking jet exhausts, for example?

What is I mean is that how does the IR system detects fire inside an aircraft?

You should add precisions. What kind of aircraft? What flame detectors? What are your current state of search? What exactly do you understand from those flame detector and fire detection systems? The more precise you are, the more specific and adapted to your current knowledge our answer will be.

Please clarify whether you want to know why fire detection uses IR sensors (a body in fire radiates heat which is energy mostly in IR), or how an IR sensor is able to detect IR (sensor technology).

I feel like the "-1" are coming from people that have never seen an infrared flame detector. "Not knowing enough to respond" doesn't mean "should downvote"

Infared flame detectors have an imaging sensor that "sees" infared light.
When a flame ignites the various  types of materials burn at different strengths, intensities, and -what we care about- wavelengths.  In the visible  spectrum  we can describe a flame as red or blue which can be simplified flames discrete wavelengths.
The IR sensor can detect the wavelengths  given off outside that visible light spectrum. This helps eliminate sources  like a technicians  flashlight from setting off a pure visible  optical sensor.
An example from Wikipedia shows a distribution  of this emission.
! Fire spectrum

There a several types of IR sensors. The simplest measure a point and radiometrically read the object temperature.  Other sensors like room occupancy sensors read the ambient temperature but don't care about the ambient temperature. Rather they have lenses which create a series of apertures which interrupt the sensing as a warm body moves around the sensor field of view. That interruption rate range is thresholded to sense the expected periodicity of the event (eg a human moving in a room).
Both types are employed in aircraft fire detection and in some other systems like gensets.
If there is a particular sensor model of interest, the manufacturer specs on that sensor will tell you if it employs a lens to detect motion.
A third variant is to sense the ambient in an area with one wide view sensor and use a second sensor to read a localized temperature in an area of interest. If the sensed differential exceeds a threshold then the sensor system shows a triggered event.
Fire detection methods such as ionization measurement don't work well in aircraft applications as they are suspectable to airflows changing concentrations and therefore the sensing threshold.

Flame Detectors
Optical sensors, often referred to as flame detectors,
  are designed to alarm when they detect the presence of
  prominent, specific radiation emissions from hydrocarbon
  flames. The two types of optical sensors available are infrared
  (IR) and ultraviolet (UV), based on the specific emission
  wavelengths that they are designed to detect. IR-based optical
  flame detectors are used primarily on light turboprop aircraft
  and helicopter engines. These sensors have proven to be very
  dependable and economical for these applications.
When radiation emitted by the fire crosses the airspace
  between the fire and the detector, it impinges on the detector
  front face and window. The window allows a broad spectrum
  of radiation to pass into the detector where it strikes the
  sensing device filter. The filter allows only radiation in a
  tight waveband centered on 4.3 micrometers in the IR band
  to pass on to the radiation-sensitive surface of the sensing
  device. The radiation striking the sensing device minutely
  raises its temperature causing small thermoelectric voltages
  to be generated. These voltages are fed to an amplifier whose
  output is connected to various analytical electronic processing
  circuits. The processing electronics are tailored exactly to the
  time signature of all known hydrocarbon flame sources and
  ignores false alarm sources, such as incandescent lights and
  sunlight. Alarm sensitivity level is accurately controlled by
  a digital circuit. [Figure 17-9]

Source: Aviation Maintenance Technician Handbook - Airframe - Chapter 17: Fire Protection Systems (FAA)

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Current FMS with COMM tunning function capability?
I am currently more familiar with GARMIN 1000 systems than with the ones that are used nowadays in airliners.
I know that you can tune a NAV aid from the FMC in an airliner. Actually, modern FMS come with the auto-tune capability for NAV aids (which helps to achieve the desired RNAV or PBN requirements). My question is if there is something similar regarding the COMMS frequencies. Let me elaborate:
With Garmin 1000, you can even select COMMS frequencies from the FMS. You can actually search an aerodrome and, if selected, it will automatically tune the COMM frequency for that aerodrome in the standby freq. Therefore:

Is there any mechanism like this one in the FMS of typical modern airliners? (a320, b737, a380...). If not, could you please give me examples of typical GA aircrafts that come with FMS with that capability?
Is there any way that we can we select comms frequencies from a typical FMS in an A320 or B737, or do we have to use always the nav/comms station to do so?

On Airbus/Boeing airliners, the Comms system is completely separate from the Nav system, so - no.
The 787 has the ability to pre-store lots of Comms frequencies, so that you've already got the numbers in the box. But it doesn't have the modern light aircraft capability to tell you that this frequency is this airport and that facility.

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Must quadcopters have the same propeller sizes?
I was flying quads today and it just suddenly popped up into my mind — do the propellers all have to be the same size in quadcopters? Mine are, but What if two propellers were larger and the other two smaller? The drone would still be symmetrical, but would it have any effect on the performance of the drone? Could you explain the physics behind your answer?

The control loop in the craft basically thinks "I'm changing attitude in such a way that I need to increase the thrust from motor #3" (for example), and it will keep increasing it until either the craft is under control, or the limit of the motor is reached (and will probably result in a crash). It may also decrease other motors to help compensate, but lift is limited. Note, however, if the control loop was tuned with one set of props, and you switched two to a different prop, the tuning will probably be off (it won't fly as well).

The main point of a quadcopter is that it can be controlled by engine power using much simpler fixed-pitch rotors¹. Assuming this construction, you need to be able to separately adjust power of

forward and rear set of propellers for pitch control,
left and right set of propellers for roll control and
clockwise and counterclockwise-rotating set of propeller for yaw control.

If you had two bigger and two smaller rotors, then one of the combinations would have the two bigger ones in one of the sets and the two smaller ones in the other. That would make it really, really hard to balance all the parameters to maintain appropriate controlability.
I think it might not be completely impossible, but it certainly makes no sense.

¹ Of course once you add cyclic and collective control, you can fly with one rotor only (well, two, or one with anti-torque rotor or jet) as most full-scale helicopters do, but then you've lost the simplicity of fixed propellers.

In addition to cyclic and collective control the full size helicopters need a means to cancel the rotation of the body. Most commonly a tail rotor.

@PeterGreen, single-rotor ones do. Twin-rotor ones don't, because the two rotors cancel each other—as long as they can have differential cyclic (for tandem rotors) or differential collective (for coaxial rotors) applied.

No, they don't have to be the same size. You could even fly with just two rotors - the Chinook does - the other pair would be needed for control only.
But having them all comparable makes it easier to design the quadcopter and keeps it generally more maneuverable.

Note however that Chinook has cyclic and collective controls on both rotors, which is something that quadcopters generally omit.

… and I think this is actually significant. You would have problems with yaw control.

It‘s also no longer really a quad copter with just rotors

Yes. But the principle of a quadcopter is varying thrust only. While you need it to be symmetrical about the yaw axis for that, it doesn't have to have quadrilateral symmetry - it's just easier and a lot more practical that way.

No, you can fly a quadcopter using different rotor sizes. The smaller rotors would have to have a longer moment arm relative to the Centre of Gravity, and as @JanHudec points out the total rotational impulse in yaw direction must zero out as well.
So two pairs of counter-rotating rotors, correctly placed relative to CoG, would work.


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What is a "natural flight loop"?
I'm roughly (edit: very well) familiar with flight simulation, but recently I came across a new term. In this brochure (scroll to the second page) by Airbus about flight simulators, they talk about

Natural Flight Loop models:
GRM, Aerodynamics...

I've never heard that term before. What does "Natural Flight Loop" mean? Is it Airbus-specific?
(GRM is probably ground-reaction model, aerodynamics is obvious - but that doesn't really explain Natural flight loop, does it?)

EDIT: Ok, "roughly familiar" is a bit understated... I should have made clearer that I'm an aerospace engineer. Actually I have been working on flight dynamics models for simulators myself, so I understand very well, what it needs to develop an accurate flight simulator from a technical point of view (yup - natural flight loop seems to be what is required for FAA FTD Level 6 devices and FFS).
The issue that remains is that I'm curious what this natural flight loop contains in detail.
Is there someone out there who can tell me if this is...

executable code (like a .dll or whatsoever),
mathematical equations (with probably huge lookup-tables for the parameters),
something like Simulink models (or Modelica or ... )?
does it contain separate models for actuators, gear, aerodynamics, weight & balance,...?
does it contain just the open loop model or also the flight control system?

At Airbus, the Natural Flight Loop is the set of the models that simulate the Natural
Aircraft behaviour: kinematics, mass, fuel, center of gravity, inertia, thrust, ground handling, ground reaction, slat/flap, aerodynamics.
Let me know if you need more info.

Ok - thank you for your answer. Can you provide more information, how these models are distributed (executables/mathematical equations/Simulink models/...)? Is this just one big model that contains all the parts or are the models separated and need to be integrated by the user?
What about the FCS - is the natural flight loop open or closed loop?

It depends on the model, but they have to be integrated by the user. What are you thinking to build?

I try to get some idea what it takes to develop the software for an FTD/FFS simulator.

I am building something similar. One thing not included in GO5 is the scnery which you have to integrate with the flight model. Send me your email if you wish to chat.

It's saying that the the sim software uses mathematical models (input/computation/output loops) that are able to accurately replicate natural aerodynamic behaviour, providing a sim that behaves closer to the real thing.  Some simulators are better feeling like the real thing than others.


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How likely could there be a loss of cabin pressure?
You often on commercial aircraft (whilst on the ground to the runway) hear on the safety video:

in the event of a loss of cabin pressure, oxygen marks will be released from the panel ....

What could cause a loss of pressure? Is it at all even possible for it to just drop?
Season's greetings :-)

According to my company's training materials, an FAA study in the 1960s of depressurization events in business, airline, and military jet transport aircraft determined that the odds of experiencing cabin depressurization were one in 54300 flight hours. I have been unable to locate that study or substantiate the statement. Either way, though the data is now outdated, it is the only hard number I have ever heard.

@JonathanWalters: That looks a bit frequent for today, there are 100,000 commercial flights per day, at least 100,000 hours of flight (and actually much more). That would be 2 pressurization incidents per day, only for airlines.

Check http://avherald.com/h?search_term=Loss+of+cabin+pressure&opt=1&dosearch=1&search.x=29&search.y=18 for an idea of how often cabin pressure is lost (and what a non-event this is thanks to the oxygen masks). Keep in mind that, even though Avherald does not report all incidents, there are over 100.000 flights a day, so it is still quite rare.

@mins I agree; I'm sure the rate has improved by now. Our training materials still require that we give the rate as 1:54300, but that is another matter.

@Sanchises The AvHerald data would suggest an improvement of an order of magnitude since the 1960's study.

Aircraft incidents are an extremely remote event (they say that you're more likely to be killed in a car accident on your way to the airport than a plane crash). Within all the incidents that engineers foresee, some are more likely than others. These incidents are included in pilot training (and passenger briefing if applicable) to increase the likelihood that it can be handled properly. Loss of cabin pressure is one of them.
Technically air pressure inside the cabin cannot be "lost", but the term is used to describe a significant drop in air pressure. The masks deploy when cabin altitude is around 14,000 feet.
Cabin pressure is provided by two methods:

The engine actively pumping air into the cabin
The fuselage is sufficiently sealed to contain air pressure inside (it is not 100% sealed, but the pump is powerful enough to overcome the small leakage)

Failure of any of these two components would lead to a loss of cabin pressure.
Historically, causes of cabin depressurization has included:

Failure of all engines, whether due to volcanic ash, fuel exhaustion, or any other reasons
Failure of all air conditioning units on the aircraft
Incorrect setting of the air pressurization unit
Rupture of fuselage
Door flew off the aircraft
Cockpit window flew off the aircraft

You may well be more likely to be killed in a car crash on the way to the airport than be killed on your flight, but the question asks how likely you are to be involved in a depressurization incident, not how likely you are to be killed.

@Emrakul The question says nothing at all about being killed and, as far as I'm aware, most depressurizations aren't fatal for most or all of the people on board. So actually there's a huge difference, and comparing the risk of dying on a plane flight with the risk of dying on a car journey tells us nothing at all about the risk of being involved in a depressurization.

While not statistically precise, a search of the NTSB accident/incident database for "depressurization" results in 31 items between 4/1988 and 8/2016 with 6 fatalities.  That's a pretty low probability, but not much consolation to the six.

@Gerry actually, some of these 31 results reference entirely different events, like loss of hydraulic system pressure, and most of the fatalities are in small private aircraft. The single relevant victim is, curiously, a flight attendant who was thrown out of the plane by excessive pressure.

@IMil Well, you can phrase it as "thrown out of the plane by excessive pressure" if you like. Assuming you're talking about the flight attendant on Aloha flight 243, it might be more natural to say that she was sucked out of the plane as it depressurized after a panel broke out of the plane's fuselage.

You can get a rough idea how common some kind of issue is by searching for it on The Aviation Herald.
In this case, you might want to search for loss of cabin pressure oxygen masks. Unfortunately, the search can't distinguish between “oxygen masks were released” and “oxygen masks were not released” and occasionally mixes in irrelevant result. Also not all incidents are necessarily reported here—there is no central registry of aviation incidents and AvHerald collects the reports from bulletins of the various accident investigation boards over the world, some of them being much more careful and detailed in reporting than others.
So from those results, I'd say there might be on order of ten or low tens of incidents where the oxygen masks release a year, worldwide.

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Why does one announce "Thrust Set"?
I have watched many aircraft takeoffs now where the flight deck announce 'Thrust Set', just after the engines are set to full.
Forgive the dumb question, but why does one announce that the thrust is set, perhaps more importantly, why is it necessary in the takeoff procedure?
I did attempt some research before posting, but the closest article I got did not make much sense to me I am afraid.

For the GE CF6 engines, N1  should be stable at 70% before the TO/GA switch is pushed to set the armed thrust limit. If TO/GA is not pushed by 50 knots, thrust must be set manually and when VNAV goes active at 400', the A/T will automatically go active in THR REF, so you don't need to select anything if the A/T is armed.

The flying pilot calls "set thrust" and the non-flying pilot sets the thrust to the precomputed amount.

@SteveKuo, the RAC's answer disagrees with you (pilot-looking-in is PM, PF must be looking out).

There was at least one incident where a Boeing 737 attempted to takeoff with the throttle levers not set to takeoff power.
The procedure is designed to ensure

the thrust lever is where it is supposed to be
the engine indications show the correct amount of power is produced

In some airlines, the procedure calls for both pilots to put their hands on the levers and advance them together. Either way, the purpose is to have both the pilot flying and pilot monitoring to ensure takeoff power is properly applied.

The main problem wasn't the throttle position in Air Florida 90. The engine EPR indication was incorrect due to probe icing. The thrust levers are set to a target EPR and were properly set based on the indicated values. The only thing that would have mattered is the pilot's "feel" for the proper position in this scenario.

@user71659 while there are no markings along the throttle positions, an experienced pilot will naturally move them to the usual position after executing hundreds of takeoffs. In the Air Florida 90 case, the throttle were advanced to the proper position, but were eased back after the (incorrect) EPR exceeded the expected value.

Qualifying Air Florida 90 (78 fatalities) as an "incident" is a bit an understatement... see https://aviation.stackexchange.com/questions/14074/ . Might want to edit that...

@user71659, these days in most aircraft either A/T that moves the thrust levers, or they are moved into a MCT/FLX detent and FMS assigns it meaning according to the programmed values, so feel does not help any more.

The "Thrust Set" call is an announcement by the Pilot-Looking-In to the Pilot-Looking-Out that the thrust is what they wanted, and what they need for the Takeoff.
In addition, the call has to come before the "80 knots" (or equivalent) call so that the proper amount of thrust has been set before there is too much ram-rise affecting the thrust setting.


text:
Did Kai Tak have a straight-in approach to runway 13 for higher-performance aircraft?
Runway 13 at Hong Kong’s Kai Tak Airport (closed in 1998 and replaced by the current airport at Chek Lap Kok) had a hideously difficult approach in which pilots would fly in from the west, following an instrument guidance system (IGS)1 to the approach’s middle marker (also helpfully marked by a giant checkerboard painted on a hillside, which, if not spotted in time, required a missed approach), and then immediately hand-fly a steep right turn2 at very low altitude directly over the heart of Kowloon City to line up, seconds before landing, with the runway, using only the Mk. I eyeball for guidance, with no room at all for error.
For obvious reasons, only the best of the best pilots were allowed to shoot the bent approach to runway 13, and only after extensive simulator training for that specific approach, and even they had considerable trouble with it, even in good weather.
The bent approach was necessitated by the presence of an inconsiderately-placed mountain in what would otherwise have been the runway 13 approach path, which, for a straight-in approach, would have forced the use of a glideslope too steep for a widebody jetliner to safely fly.  Not all aircraft (or even all airliners) are widebody jets, though, and smaller, higher-performance aircraft (including virtually all STOL aircraft) can frequently shoot approaches much (much) steeper than what would be safe for a 747 or A330.
Given the difficulty and hazard of the standard bent approach, did Kai Tak have a straight-in approach to runway 13 for use by steep-approach-certified aircraft?

1: Like an ILS, except that following the IGS all the way down would kill you.
2: I don’t know if the Kai Tak checkerboard turn would technically qualify as a pylon turn, but it must surely have come close!

The reason for the off-axis approach was a rather inconveniently placed mountain, so there was no way to make one. Also, 747 is a high performance aircraft, the deciding criterion for this purpose being the landing speed.

"inconsiderately-placed mountain" I must complement you on your writing. While some of your questions have been closed, they've all been entertainingly well written.

did Kai Tak have a straight-in approach to runway 13 for use by steep-approach-certified aircraft?

During the 1990s I regularly flew the Hong Kong IGS approach in 747s. At that time, to the best of my knowledge, there was no straight-in approach to runway 13.

only the best of the best pilots were allowed to shoot the bent approach to runway 13, and only after extensive simulator training for that specific approach, and even they had considerable trouble with it, even in good weather.

While the above might have been true for some carriers, it was not true (Wikipedia to the contrary) for the two 747 carriers I worked for. Cathay Pacific might have had some such requirement as that was their home base.
Go to https://terryliittschwager.com/talking-of-flying.php#kaitak for an account of my first time in.
Wikipedia has another error or at least a poor choice of words IMHO when they say that upon reaching the checkerboard hill you started the turn toward the runway. if you look at the approach plate, you'll see the turn starts well before the checkerboard. The rule was that you had to have the checkerboard in sight ahead before starting the turn. If you didn't it was missed approach time. Fortunately I never had to do that.

to the eternal gratitude of non-masochist pilots everywhere) had a hideously difficult and dangerous approach

I never considered the approach dangerous. Challenging, yes, but not dangerous. And I'm really not a masochist or at least I didn't think of myself as such. After you got used to it, it was fun!

Elsewhere on the internet, it is customary to warn people when a link leads to tvtropes.org because it can be a huge inadvertent time sink. Your site is the aviation equivalent :)

@AEhere – having just read every anecdote on it, I agree with you.
Given that Terry doesn't think that this special approach procedure was "that bad", why is that we've never seen it anywhere else  -- did technology improve everywhere else, or was it just the geopolitical uniqueness of Hong Kong that made it a one-off?

@Landak It's geopolitical; Hong Kong is a bunch of mountainous islands, and Kai Tak was put on one of the only large, flat surfaces available within the colony. They had no alternative at the time. Without the political issues, they would have just put the airport on the mainland.

It’s a sad thing they no longer maintain the checkerboard pattern.

@vasin1987 I always thought it would be nice to have a park around it, and with so much history involving Kai Tak, it could make a memorial of sorts.

I believe the main issue why a straight in approach was not feasible was the proximity to mainland China border to the north and west.. less than 30kms to the north if I remember correctly. Also Macao (also to the west) had its own airport and approach/departure patterns complicating matters. You need to check and older map which shows the boundaries of Macao and Hong Kong to get an idea of how tight space really was.
A Glideslope of 3 degrees is normal. 3.5degress occurs and some 4 degrees (RWY31R MRS for example). What is desired is a rate of descent not more than 1000ft/minute and jets cannot really do that easily on anything more than a 3.5 degree glideslope with autoland. I believe London City (LCY) has a Glideslope of 5deg but I have no experience of that.
It is not true that only the 'best' were allowed to fly the Rwy13 approach, a lot of airlines practiced an RQ or 'route qualification' where a pilot needed to fly an approach under supervision of someone who has been there before. Even First officers are given landings there.
Alan Dow.. What usually happens when a pilot unfamiliar with HKG flies in is that the tenancy to start the turn the moment you see the runway on your right is very strong .. remember the checkerboard is coming up real fast! so you make an early right turn (1000-1200ft AGL) and then realise the runway is to your left and you need to get your shit together as you're running out of time, distance and altitude so its a hard bank to the left to get lined up on the runway...hence you see airplanes making all sorts of messy landings.. Alitalia, Malaysian and China Airlines have videos/pictures of how messy it gets. Every pilot new to HKG was briefed on not to make the turn too early but most people did anyway.
Terry.. HKG being the homebase of Cathay, I would imagine all their pilots including F/Os could do it in their sleep as HKG would be every other landing for them.
For those who are interested, this is the checkerboard today..
https://goo.gl/maps/DdpgGJzUrqEvbBXe8


text:
Why are Gatwick's runways too close together?
Gatwick Airport has two runways (08L/26R and 08R/26L), but they are only 200 metres apart - too close (by just ten metres) to allow simultaneous operations on both runways.  As a result, usually only 08R/26L (the older, more southerly, and slightly longer of the two) is used as a runway, with 08L/26R serving only as a taxiway except when 08R/26L is closed for some reason.
Given that Gatwick has plenty of space available that could have been used to put 08L/26R ten metres further north and thus allow simultaneous operations on both runways, and that Gatwick is, and was, bursting at the seams with the amount of traffic it has to fit down 08R/26L, why was the new runway built just slightly too close to the original?

Gatwick was made in the 20's and developed in the 50's and 60's. Back then the aircraft weren't big enough, nor was there that much traffic. So really this is probably just about available space and historical decisions not being able to foresee the size/frequency of modern air travel.

This sounds like a similar question as to why would they have built Stonehenge so close to the A303.

@Neil : Nope, because the Stonehenge doesn't serve a practical purpose and is old enough to be a protected historical site. If it was newer, and it had some important practical purpose, the question when they'll relocate it, would make sense.

@Neil: Not unless Stonehenge was built in the 1970s.

How do you know, @vsz, that Stonehenge doesn't/didn't serve a practical purpose?

Historically, Gatwick had only one runway (08R/26L) with a taxiway to the north, but this obviously would cause problems if that runway was out of use for any reason - emergency, scheduled maintenance, etc.
In 1979 the taxiway was upgraded so that it could be used as a runway when the main runway was out of use. Since this used existing infrastructure the locations were obviously fixed.
However, at that time the then owners of the airport (BAA) agreed with West Sussex County council that the second runway would only be used when the main runway was unavailable. This was to protect the interests of the airport's neighbours. With this agreement in place there was no incentive to address the issue of separation. The agreement was to run for forty years, and expires* on 13th August 2019.
With the imminent expiry of that agreement the airport is now exploring ways to  bring that second runway into use. One suggestion is to widen the existing main runway southwards, allowing the centerline to be moved south and increase the separation.
Source: The Independent, 15 October 2018

Note: at the time of writing the agreement was due to expire just three days later. Now (March 2022) it has indeed expired.

Quite a coincidence, this question coming so close to the expiration date of the forty-year agreement!

@PeterLeFanuLumsdaine: Quite a coincidence indeed, but a coincidence nonetheless - I only found out about that agreement while I was doing research for this question (and still didn't see why they couldn't have - say - added pavement to the north edge of the ex-taxiway in order to move its centreline sufficiently far away from that of 08R/26L, hence me still asking said question).

Obligatory: "There are only coincidences"

Edit rolled back to my original text. The change from 'expires' to 'expired' made nonsense of the following paragraph. I've replaced that edit with a footnote.


text:
How much of an effect do structural overloads have on an airframe's safe fatigue life?
Most aircraft1 are built primarily out of various aluminium alloys.  Aluminium and its alloys have multiple physical and chemical properties that make them terrible choices as structural materials, except in situations (such as the primary structure of aircraft) requiring the excellent strength-to-weight ratio of many aluminium alloys.  One of these unpleasant properties is that they have no fatigue limit,2 so that any change in stress magnitude or direction, no matter how small, will produce cumulative damage to the material.  When used as part of an aircraft's primary structure, this means that the airframe has a hard limit on how many times it can be stress-cycled3 before the cumulative effects of fatigue damage render the aircraft unsafe to fly unless repaired.
Occasionally, aircraft encounter circumstances that place greater-than-usual stresses on their primary structure, such as hard landings, encounters with extreme turbulence, or overly-aggressive maneuvering.  These aggravate fatigue damage in two ways:

The abnormally-large amplitude of the stress cycle during which the structural overload occurred causes the additional fatigue damage accrued during said stress cycle to be greater than that accrued during a more-normal stress cycle.
Large overloads can directly produce small-scale cracking and buckling within the aircraft structure, which (as they produce stress concentrations in whatever structural element they occur in) can serve as initiation points for progressive fatigue cracking.

How much of an effect does a structural-overload event have on an airframe's remaining safe fatigue life?  Are there any rules of thumb that say that (for instance) a hard landing takes off X number of takeoff/landing cycles from the remaining fatigue life of the hard-landed aircraft?  Are there qualitative effects on the airframe's fatigue life that I hadn't thought of?

1: Despite the best efforts of both Boeing and Airbus in this regard.
2: A minimum level of applied cyclic stress below which the material in question does not experience fatigue damage even if stress-cycled indefinitely.
3: Stress-cycling takes different forms for different portions of the airframe; for instance, the wings care about takeoffs and landings, when they have to suddenly start or stop carrying the weight of the rest of the aircraft, while the pressure cabin cares about every time you climb or descend to, from, or between altitudes requiring that it be pressurized, which causes it to start, stop, or change the degree to which it's holding in a lot of air that would really like to get out.

Please post answers as such, and keep comments for clarifications.

Airplane structures are designed to take a certain number of extreme conditions during their life which are defined based on statistical analyses of past flights.
For example a manufacturer can design the structure expecting (just random numbers) 10 minutes of flight in extremely severe gusts, 80 minutes in severe gusts, 5 very bad landings, 20 bad landings and so on. This kind of statistics are obviously confidential and possibly different among manufacturers.
If an unusual circumstance happens, the manufacturer is informed and appropriate measure are taken. Most of time it's something that can be easily corrected using standard procedures otherwise specific analysis are performed.
FAA has a very nice section about past accidents happened due to fatigue problems

As a side note, structural aeronautical aluminium has the same strength-to-weight ratio of structural aeronautical steel. It has been chosen over steel because it is easy to work, doesn't rust (more or less) and everybody can learn to correctly rivet in a couple of minutes. There's obviously much more than that, but that would be another question.

"Structural aluminium has the same strength-to-weight ratio of structural steel." You're probably looking at the wrong alloy, like 6061, which isn't used structurally. You need to look at the 2000 series like 2024, which is over 2x the specific strength of steel. 2000-series alloys have poor corrosion resistance, so you have Alclad, which is pure Al bonded to it. 7075 is also popular because of better corrosion, it's used in planes and cars.

Welding has better structural, mass performance and is cheaper than riveting and similar fasteners. Aerospace fasteners get very expensive. You don't normally weld aluminum because it's very easy to ruin the mechanical properties and microstructure. It's a downside, not a benefit. Aircraft manufacturers ended up developing specialized techniques like laser welding and friction stir welding to get around this.

@user71659 I was just looking at 7075 vs. some aeronautical steel alloy, they do have indeed very similar ratios. Never spoke about welding or about pros and cons of the several existing ways of bounding structural pieces together, anyway thanks for your clarifications, you should put them in a new q/a

A single overload that doesn't exceed the yield limit on the structure has no effect. It's a spring; as long as you didn't permanently bend it, it's good.
This is why when hard landings, or turbulence encounters occur, the resolution is an inspection to see if any part of the structure has yielded, torn or cracked. If not, it's good to go and there isn't any service life penalty.
I know of an RJ in service in Scandinavia that had a bird strike that took out some fan blades and vibrated the back end of the airplane so violently the flight attendant strapped in back was injured from having her arms and legs flung all over the place.
They inspected the plane, nothing was permanently bent, and it was returned to service.
I know of another RJ that was overstressed in a pre-delivery production test incident. It was inspected, nothing was bent, and it was delivered to the customer.

Even a single overstressing accident can have a disastrous effect on fatigue life. For example, the 747 that crashed in Japan some years ago, claiming over 500 lives, had been subject to a runway tailstrike due to over-rotation a few years before that which significantly overstressed its rear pressure bulkhead. Field repairs were carried out to strengthen it, but were improperly executed- and the bulkhead's weakened state persisted until the microcracks in its structure grew far enough to cause the bulkhead to explosively fail.
ONE tailstrike!


text:
What disadvantages does flying a drone close under a ceiling have?
If I want a drone (quadcopter) to be flying always under a surface and close to this surface (i.e: under a table 5cm away from the surface), which are the disadvantages this would have in the lift generation of the drone?

How much would be the lift lost due to this flying conditions (in percentage or with calculations if possible)?

Why do you think lift would be lost? Not enough air available to push down?  I would suspect that may be dependent on the drone size if it came down to that.  I'm not a good enough drone pilot to get that one that close and be steady with it.

@CrossRoads that is why I am asking this question, I do not really know if lift is lost or not. I suppose that the air mass flow is lower, as there is interference with the surface above. So the purpose of this question is to answer the question you asked me.

Tons of body styles too, that's got to have some impact on how airflow is available to the top of the rotors. You might have to do some modelling for a specific craft to get an answer. https://www.amazon.com/Hobby-RC-Quadcopters-Multirotors/b?ie=UTF8&node=11608080011

For a quadcopter, flying close underneath a flat surface will increase the amount of lift produced, but this will make it harder to control (unless the quadcopter is equipped with appropriate control software).
According to comments by Alex and Peter Kämpf, if you fly a quadcopter close underneath a flat surface, the quadcopter will tend to get "sucked up" and collide with the surface. You will have to reduce power dramatically in order to go back down.
You could probably make a quadcopter under a ceiling easier to control by equipping it with a proximity sensor (or perhaps an accelerometer) and software which automatically reduces power when the aircraft gets close to a ceiling. It could take a lot of tuning, but with the right control algorithm, it would probably be easy to maintain a constant distance under the ceiling.
So, why does a quadcopter behave this way? I don't know if any research has been done on this, but I can make an educated guess.
For aircraft flying above a flat surface, the ground effect increases the amount of lift produced. When the wings are near the ground, a high-pressure region is created between the wings and the ground, thereby increasing the amount of lift.
So for aircraft flying below a flat surface, presumably something similar happens. The propellers create a low-pressure region between themselves and the upper surface. That once again increases the amount of lift produced.
(In the extreme case, with an object which is in contact with the upper surface, it's possible for the object to create a low-pressure region above itself, and thereby keep itself up, using no power at all. That's what a suction cup is.)
For posterity, here are the comments by Alex and Peter Kämpf that I referenced above:

I can say that when I fly my smaller sized quads indoors that its more of a suction cup response. The few that I have all have generally the same effect. If I fly to close to the ceiling I end up sucked up to it where then I have to dramatically drop throttle before they are released. It takes a surprisingly little amount of power to maintain the suction effect. Keep in mind they all have duct motors with a flat surface on top (no camera or antenna sticking in the way) so that may provide more suction action as well. – Alex 2019-01-16 15:50:56Z
There is an important difference: While 'regular' ground effect is dynamically stable, this inverted ground effect is the opposite. Coming close to the ceiling means that the quadcopter will be sucked up until it collides with the ceiling. – Peter Kämpf 2019-01-16 16:26:17Z

I can say that when I fly my smaller sized quads indoors that its more of a suction cup response. The few that I have all have generally the same effect. If I fly to close to the ceiling I end up sucked up to it where then I have to dramatically drop throttle before they are released. It takes a surprisingly little amount of power to maintain the suction effect. Keep in mind they all have duct motors with a flat surface on top (no camera or antenna sticking in the way) so that may provide more suction action as well.

There is an important difference: While 'regular' ground effect is dynamically stable, this inverted ground effect is the opposite. Coming close to the ceiling means that the quadcopter will be sucked up until it collides with the ceiling.


text:
Is it usual for a first officer not to know how to taxi?
According to this article on afspc.af.mil, the first officer of the 737 the pilot in the story was flying with had never taxied a 737. Is this normal?
The exact quote in the article:

After landing, the first officer turned to Gongol and asked if he knew
  where to taxi, she had never been to the Omaha airport before. Taken
  aback by how cool, calm and collected the first officer had acted
  without knowing the airport, Gongol remembered landing at the airport
  before pilot training.
"Surprisingly, taxiing was the most stressful part of the day for the
  first officer," said Gongol. "She had never taxied a 737 before and
  the ATC had no idea that the pilot was the reason for the emergency.
  We had to make a quick decision that her switching to the pilot's seat
  and taxiing the aircraft without the training was necessary to save
  the captain's life."

"never taxied" != "doesn't know how to taxi"

According to the story the FO had never taxied a 737, never been at that airport before and was "taxiing the aircraft without training". I suspect this means the FO had taxied other aircraft types at other airports and did "know how to taxi" in general, just not this aircraft type at an unfamiliar location.

In some airplanes, there is no tiller on the first officer side, so they are unable to taxi.  Even on those that do have a tiller on both sides, it often a captain-only duty per the CFM.

@casey: this answer might be relevant, especially the point about captain overriding the company policy of not allowing F/O to do this or that.

@casey Here is a picture showing a tiller only on the left.

After reading that article carefully, here's what appeared to have happened:

Pilot collapses
Passenger-with-pilots-license comes to flight deck (with both elitist and sexist chip on his shoulder)
First officer moves to left seat, PwPL takes right seat, runs the radios and checklists, maybe drops the gear.

First Officer is a right-seat qualification, on 737s (and many others) the steering handle is on the left side.
So it's quite possible that the FO has never steered a 737 on the ground. It's quite IMpossible that the FO has never taxied an aircraft of any kind before. And there's a huge difference between driving a Gulfstream around and driving a 130-passenger jet around.
If you don't take a corner just right, the main gear ends up in the ditch, the wing hits the ground, the engine inhales grass, gravel and maybe the taxiway sign and a couple of gophers. Trashing a Ferrari is cheaper.
The FO moving to the left seat and flying the plane was the correct action - the B-1 pilot was no more qualified to fly a 737 than I am. But radios are radios, and even the greenest flight attendant can read a checklist.

Chip-on-shoulder-wise, I particularly like "we made the decisions". That is to say, the FO made the decisions how to get the plane down, and the USAF pilot made the decision not to hijack it. Also, "ATC had no idea the pilot was the reason for the emergency". Whose fault is that, Capt. operating-the-radio? ;-) Not to say he didn't do the right things, but I wonder if the journalist has got a bit carried away with the military here.

@SteveJessop, the author of the article is a member of the Air Force, so I suspect that sort of spin is normal.... for them.


text:
What is the role of HP & LP fuel pumps in aircraft?
What is the role of HP (high pressure) and LP (low pressure) fuel pumps in aircraft like the A320? Why are two different ones needed?

There are no HP vs LP pumps in an A320. The various pumps supply the same pressure. Some models have jet pumps that use electric pump pressure to transfer fuel. Where did you see mention of two different kinds of electric pumps on A320?

https://hursts.org.uk/airbus-technical/html/ar01s19.html
It's shown in the design diagram.
It talks about how the engine master switches control the LP & HP shut-off valves, can you please elaborate what exactly are they?

@JuanJimenez Clearly, you can see A320 has HP & LP pumps in edit :) !

You need HP pump to push fuel into the combustion chamber against the chamber pressure, and also power the hydraulics of the engine. But the HP pump is at the engine and quite far from the tank, so you really can't rely on it to suck fuel from the tank for itself, unless you pressurize the entire tank. This is just like a car: you have a LP pump at the tank and a HP pump (or two) at the engine.

I think these pumps are in the engine.  So more of a question about why the engine needs it than the airframe.

@user3528438 But why one is "low pressure" and other "high pressure"?

@SachinChaudhary The way I interpret your diagram is that there is one type of pump that provides fuel to the engine and to the oil cooling system. That pump is low pressure. Inside the engine there is another pump that picks up this high volume, low pressure fuel and sends it to the turbine injectors at high pressure, but those pumps are not technically part of the aircraft's fuel system, they are part of the engine. This is the same as in my BD-5J with a Microturbo Couguar engine. low pressure pump out of the tanks, mechanical high pressure in the engine driven by the accessory drive.

Please add a reference for the picture.

its in fcom....

In the schematic, you have marked the LP and HP shutoff valves, not the pumps. Is your question about the valves?

The question asks about all low pressure fuel pumps on commercial aircraft but the diagram depicts only one specific instance (of a two stage high pressure pump) and is confusingly labeled.
The low pressure pumps move fuel around the aircraft where only volume is needed. These pump through comparatively large pipes with low back pressure, so they operate at less than 60psi. Fuel is used to maintain balance, reduce flutter susceptibility and for cooling in addition to providing an energy source for the engines. The a320 has six low pressure pumps serving the center and inside fuel tanks. A series of valves control where fuel goes. The outer tanks gravity feed the inner tanks, so no pumps are needed.

The high pressure pumps are attached to an engine and atomize fuel into a burnable spray inside the combustion chamber, where high pressure is needed to overcome combustion pressures as high as 600psi in modern engines. This nozzle test is at 200psi but in operation the pressure may be 1000psi or higher.

The high pressure pump typically consists of multiple stages as a single stage cannot achieve the desired pressure. Shown is a two stage high pressure pump with a low pressure centrifugal boost stage on the right followed by a high pressure gear stage to the left.

This matches the OP's diagram depicting a single mechanically driven engine fuel pump (in red) containing a centrifugal low pressure and geared high pressure 'pumps' (in yellow) more easily understood as stages.

There's some confusion here. The LP fuel pump is in the engine and is mechanically driven. As you can see from the FCOM schematic, the LP fuel pump is after the LP fuel valve. The pumps you're referring to are the tank pumps, they're electrical and sit in the tanks.

@Pilothead That's my point about the LP valve. In your first picture, you can see that the 6 tank pumps are upstream of the LP valve. In OP's schematic, the LP fuel pump is downstream of the LP valve. That shows they are different pumps. If you look at the A320's overhead fuel panel, those pumps are marked as e.g. "L TK PUMPS 1/2".

@user71659 So two different points: one - the question was not limited to the diagram and specifically stated a "canonical answer" about low and high pressure pumps was wanted. This requires inclusion of the pumps I described. I am not saying they are the one in the diagram. two - the low pressure 'pump' in the diagram is the low pressure stage of the two stage high pressure pump in the engine, with confusing labeling. The diagram draws a box around both 'pumps' and calls it a pump. If you have any documentation to the contrary, I'd like to see it in an answer.

@Pilothead You're trying to say the two pumps marked in one red box are a ONE single unit? I think they are their own distinct and individual pumps and not "Stages" on a one pump.

@SachinChaudhary Marked in one red box labeled "Fuel Pump" in your diagram. This whole discussion is immaterial to the question you asked, which is role.

@Pilothead you explained that hp pumps are needed to atmomize fuel ... But I can't make out from the answer why are "lp pumps " needed at all?

In general LP pumps supply fuel to the engine from the fuel tank. HP pumps supply fuel to the injectors. This principle works not only for aircrafts but even on modern cars. It's sometimes possible for engine to work if other pump is not working but with less reliability usually if LP pump is broken. HP pump at the engine ensures that you don't need long lines to have high pressure. If you don't have LP pumps you can easily run into vapor bubble issues in the fuel line. If LP pump fails engine still can use suction feed from HP pump.

You said LP pump supplies fuel to the engine, and HP pump supplies fuel to injectors. What do you mean? What's the difference? To what part of "engine" LP pump supplies fuel..? Fuel injectors are also a part of engine.... What do you mean? You're saying same thing essentially

@SachinChaudhary. Fuel generally moves around the airplane at comparatively low pressure ( < 60 psi).  At the very end of its journey, as it's about to go into the nozzles, the pressure gets boosted far higher (in a 2-stage process that Pilothead described, up to hundreds of PSI).  That boost comes not outside the engine but by a part of the engine, the HP fuel pump. Read the 1st two sentences of this answer understanding that the LP pumps are in/near the fuel tanks, but the HP pumps are in/part of the engine. Does that make more sense?


text:
Why is a level off required for flap retraction after a single engine failure?
In the Airbus A320, why do we need to level-off for flap-retraction after a single engine failure?
Even if we maintain a climb, the airspeed will still accelerate and be enough to retract the flaps safely.

Are you sure you can still climb and accelerate at MTOW?

@Bianfable the airspeed will increase for sure, why not

if the engines are working

You just said "after a single engine failure" in your question. Now you say "engines". Please clarify, how many of the two engines are working?

@SachinChaudhary You say "the airspeed will increase for sure, why not", but how do you know? What is your source? Are you an Airbus test pilot? How many times did you have an actual engine failure on takeoff close to MTOW in a real A320?

In a nutshell: the aircraft has lost 50% of thrust capacity, and you can either choose to make a shallow climb, or accelerate in level flight to retract the flaps. Unless the aircraft is very light, you cannot do both at the same time. The performance is just not good enough.
You might even decide to increase the thrust on the remaining engine to TOGA, because when the aircraft is very heavy, it hardly climbs or accelerates if you keep it at FLEX/MCT.
Additionally, you need to accelerate to certain minimum speeds first (F or S speed), and then retract the flaps step by step, which then reduces drag and makes further acceleration easier.
You state that after a single engine failure, the aircraft would still accelerate while climbing. Where does this experience come from? A professional simulator, or a simple aircraft model in something like the Microsoft Flight Simulator? Could it be that your model is just not accurate enough, or you have made unrealistic settings like payload and fuel which makes the aircraft unusually light?

The climb segments defined in the FARs or European equivalent are used for certification purposes.  The different climb segments are designed to allow the engineers the ability to easily calculate performance during a climb.

First segment starts at rotation and ends at gear retraction.  There is a requirement for a positive rate of climb during this phase of the climb.
Second segment starts at gear retraction and ends at 400' AGL.  The requirements is to have a 2.4% climb gradient.
Third segment starts at 400' AGL where the aircraft levels off and accelerates to VFTO (final takeoff speed with flaps up).
Final segment starts after reaching VFTO and flaps are retracted and ends as the aircraft climbs to 1,500 FT AGL.  The requirement is to have a 1.2% climb gradient.

Many aircraft procedures are designed around these requirements.  Airbus aircraft will level off to accelerate to VFTO and bring the flaps up so they know what performance they have.  It may be possible to lower the nose a little bit and start accelerating while still climbing but it would be unknown what sort of performance they would have.  If there are no obstacles, it may be a good idea to do that.  Leveling off, accelerating and bringing up flaps reduces drag and an increased climb rate may get the airplane to 1,500 FT AGl quicker than other procedures.

Other types can make low rate climbs while accelerating on one engine. eg B767 SOP is Vertical Speed +200 at low and medium weights once terrain clearance has been achieved. At heavy weights VS +100 is used. I guess if near MTOW at high density altitudes level flight acceleration may be necessary. Pilot’s judgment was used, not specific weights at my old company.


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For the recertification flight(s) of the 737Max, will the FAA want to see a live runaway stabilizer?
With Boeing ready to start the testing Monday, will the FAA want to see a simulated runaway trim failure in flight?

If the failure effect constitutes hazardous criticality, it usually won't be done in flight.

As far as I've seen, the FAA has not published the exact plan the test flights. The only quote I could find is "Testing is expected to take several days, and will include a wide array of flight maneuvers and emergency procedures to enable the agency to assess whether the changes meet FAA certification standards." (CNN).


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Why is only one rudder deflected on this Su 35?
Why does the aircraft has only one rudder deflected in this picture?

Because the pilot is only stepping on the right rudder pedal.

@Michael -  Look closer. You can see the left rudder is deflected. It's just deflected outboard. They do this as a speed brake, not from rudder pedal depression. unlike Rutan canard aircraft, where each rudder pedal is independently hooked to only one rudder, and the rudders only deflect outboard, the rudder pedals in twin tailed fighters (like F-15, F-18, Mig-29, etc.), are mechanized the same way they are in a single tailed aircraft. The flight control computer actually controls the rudder surfaces, based on multiple inputs.

Even prior to "Fly-by-Wire" systems, as in the F-16. fighters often had integrated flight control systems that allowed the flight control surfaces to move based on more than one pilot control input. In early F-15s, for example, left-right deflection of the flight control stick caused both deflection of the ailerons, and differential stabilator, in varying degrees depending on the fore-aft position of the stick. When it was forward (implying Low AOA), the ailerons were deflected. As it moved aft, (higher AOA), less and less aileron, and more and more differential stabilator was generated.

Can you attribute the image correctly (i.e mention the source and photographer). See  also this meta post for more info.

@CharlesBretana, I actually intended that comment as a joke.  But, your comments would make an excellent answer!

@Michael, my bad!  Add an exclamation point or a [\grin] or something to make it obvious...

Trimmed the image and edited to actually be a complete sentence with a question.

Both rudders are deflected, but from that angle you can't see the other one.  They are pointing outboard, as speed brakes.  (This image looks similar enough to some clips in a promotional video by the Chinese Air Force, described by the Xinhua News Agency, to suggest that that the photo was taken from a large Chinese military aircraft, thus by a member of the Chinese Air Force.  But I haven't found an exact attribution.)
Here's a video of a scale model operating them simultaneously with the dorsal speed brake.
Here's a video of the rudders pointing outboard during approach and landing.
The Su-35S is piloted by Yuri Vashchuk,
at the MAKS 2015 air show, Zhukovsky, Russia.
Filmed by Konstantin Khmelik.

Can you attribute the image correctly (i.e mention the source and photographer). See  also this meta post for more info.


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Could a Continental R-670 piston engine run with regular car gasoline?
I know that the minimum octane for the R-670 is 65 octanes (W670-K variant). I also noticed that other variants increase compression ratio and power using a minimum of 80 octane. Why there is an increment in octane? My guess here is to prevent detonation.
Could a Continental R-670 piston engine work with regular car gasoline (85-97 octane) or Avgas (100 octane)?
My guess is yes, but would it develop some other problems as time goes by?

Yes the higher octane is specified because of the higher compression ratio.
An R-670 would run perfectly fine on 94 octane mogas (almost all Regular car gas has alcohol - you need to run alcohol free Premium), and would run fine on Avgas except for lead fouling over time that plagues 80 octane engines run on 100LL.
The only benefit to the engine that avgas provides, other than the lower vapour pressure and therefore resistance to vapour lock, is that a combustion byproduct compound from the lead forms on exhaust valve seats that has the effect of lubricating the interface between the valve and seat (there is always a bit of sliding motion as the valve centers itself in the seat on closing), reducing wear somewhat.
Otherwise it will be happier on mogas than avgas. Supercharged  P&W R-985s are approved to run on mogas with the Petersen STC and I imaging the Continental is as well.

It is possible to put hardened valve seats in an auto engine to "upgrade" it from leaded to unleaded gas. I'm sure that could be done to an aviation engine, as well. Whether or not the engine would then be legal to fly is a whole different topic...

Cars went to seats made from a harder alloy called Stellite back in the 70s as unleaded gas came on line.  I believe aircraft cylinders also do these days, but not sure.

In the design of a piston engine running on gasoline, the required octane rating for a particular engine will depend on the compression ratio and the cylinder head temperature (which varies according to how hard the engine is working). High compression ratios and high power settings require high octane fuel to prevent preignition, which damages the engine.
Taking the old-school air-cooled VW flat four as an example, the engine's compression ratio was set at 6:1 so it could run slightly hot on the low-octane (less than ~70) car fuel typical of the late 1930's. This worked out OK because the engine was not continuously run at full power in a passenger car.
Upping the compression ratio to 10:1 or even 11:1 in a car required 100 to 120 octane gas and water cooling, and this was OK again because the car engines did not run continuously at full power. This development occurred after WWII when high-octane gas (originally developed for aircraft engines) hit the car fuel market.
Since airplane engines run at high power setting for long periods of time and have air cooled heads, they will necessarily 1) have relatively low compression ratios (around 6:1), 2) have minimum octane specifications placed by the engine manufacturer on permissible fuels, 3) run rich at takeoff power to scavenge heat, and 4) still have 3-minute time limits on maximum power.

That last paragraph is just plain wrong.  Many aircraft engines run on lower octane than modern automobile engines, comparable even to what a lawnmower requires.  Yes, it's due to low compression that they can do so, but "highest-octane gasoline available" is just not the case.

@ZeissIkon, will edit.


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What values can/should be used for the isentropic efficiency of a compressor and turbine?
I was trying to model an turbofan aircraft engine using Matlab and Simulink. In the process I have fixed certain parameters to some specific values which are nominal operating values of the parameters. Can anyone please help me out with the values of isentropic efficiencies of compressor and turbine used in the above case?

I'm no aircraft designer, but when you say "used in the above case", I'm not seeing a use case with enough details to get a good answer. Maybe you've provided everything needed, but I'm going to bet more details are necessary.

Unless you want to model a specific engine, standard numbers are given in the book 'Mechanics and Thermodynamics of Propulsion'. You can find it online. Or, you can look at this site https://www.particleincell.com/2014/turbofan-calculator/

Thank you, will look into tat

An answer is in the table of the linked question: What is the efficiency of the turbine of a modern jet engine?. Column 4 is for modern engines, column 1 is for old engines, about 1960’s technology. So, take your pick. In order to get a simulation running, an exactly correct figure isn’t necessary, something in the ball-park is fine.
The third comment above gives another similar source. Either would be fine.

Thank you, will look into that


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At which airports has the A380 landed?
I can't find a list anywhere (other than regular flights), but I am really interested, which airports have ever handled an A380 flight, including all one-time visits or emergency landings.

Airbus claims that:

The A380 is compatible with over 140 small and large airports for
  regular service worldwide, and up to 400 airports when adding
  diversion airports ...

The list of airlines operating A380s and their destinations are listed here.
I didn't find details about emergency landings.

A lot of Airports can "technically" land an A380, all you need is the 3000 or so meters of runway required. Actually servicing them is another question. For example, Halifax Stanfield (CYHZ) has just enough runway length to put down an A380 assuming they have good weather and dry tarmac, but none of it's jet bridges are capable of reaching the cabin door. Apparently, the plan in place should they need to de-plane an A380 is to call Halifax Fire Department, and for them to take their cherry-pickers to get people down one at a time.

The Aviation Herald has a pretty thorough listing of any mechanical incidents that occur in commercial aviation.  Searching that site for incidents involving an A380 yields the following diversions to airports that do not normally see regular scheduled service:

Shannon
Goose Bay
Muscat
Perth
Colombo
Warsaw
Baku (after being informed that Ashgabat, which was closer, was "not suitable for A380s")
Ottawa
Jakarta
Hyderabad (after being unable to land in Chennai due to runway length)
Honolulu
Nadi

Note that Aviation Herald does not routinely cover diversions due to weather or medical emergencies, so this is not a complete list of everywhere an A380 has ever been diverted to.


text:
Would a piston or turbine engine be best suited for a small UAV flying 8000miles?
I am trying to devise a concept that would airdrop medical supplies around remote regions of Africa: vaccines, medicine, blood... etc. I image the payload would be anywhere between 1-2 ton. I think a UAV would make the most sense because then you don't need to worry about flight-time, speed, pressurization, bathrooms and pilot logistics.
Range is an issue because I might need to determine where to set up runways. Africa spans thousands of miles. Ideally, 8000 miles would be great because you can take off from Europe or Egypt or Israel , then fly anywhere in Africa and return without landing to refuel. But landing at a hub to refuel would double the distance -or half the required range.
Payload weight is something I think about a lot and I need to do more research. Like, how light can you make vaccines? There would also be the syringes and other medical supplies. Also, because of the cold-storage cooling and parachutes I think 1-2 ton is a good estimate.
I was looking at electric powered options and these aircraft are typically short range and expensive. Unless you look at the Impulse 2 which has infinite range, but was $128 million to develop.
So in short, long range, light-ish payload, speed doesn't matter and as cheap as possible.

1 ton for 8000 miles you are in the territory of pretty advanced military drones, somewhere between MQ-9 Reaper and RQ-4 Global Hawk, or CAIG Wing Loong II. Looking at those gives you an rough idea of what the end product could be. This idea is actually very interesting, meaningful, yet surprisingly feasible. You may just buy the Wing Loong  with cash and start the operation right now. CAIG are definitely interested in humanitarian applications of their product.

Flying all over Africa from a base in, say, Egypt might look to be logistically simple, but bear in mind that different countries have different views on the legality of UAVs, particularly those that intend to drop a ton of more of stuff on their populations.

Those are 225 million per unit. I need cheap!

Speed doesn't matter? A Boeing 777 needs 16h for a 8000mi mission, so you can do one per day or 300 per year (maintenance downtime). Is this enough? I think it is much better to deliver suppliers by ship or commercial air freight and use airdrops only for the last mile.

You could go to airliners.net. (I know I know, I keep telling this to people)

We already have great things called airplanes which take relief supplies all over Africa, what are you trying to do differently?

I dont think a commercial 777 can airdrop large cargo. Also, this would need to be routine as medicine and vaccinations are constantly evolving.

8,000 miles is quite the increase from your last post asking about 2,000 miles https://aviation.stackexchange.com/questions/74810/can-you-fly-2000-miles-on-an-engine-that-costs-less-than-500k-if-so-what-type

Does this answer your question? Can you fly 2000 miles on an engine that costs less than $500k? If so, what type of engine?

Don't forget, South Africa is quite the civilized nation with a variety of airports that you could use as a base to cover the southern portion of the continent. That may enable you to reduce range requirements by being able to fly to the middle from both ends, instead of having to fly to (and return from) one end from the other.

@FreeMan, not just South Africa. Outside the four or five regions with active civil war and maybe one or two more defunct states, all the other countries have airports with regular airline service from all major world hubs. The problem is getting into the regions in need. That never requires more than 1000 km, probably much less. What it often does require though is an armed escort.

Not to burst your bubble, but doing this "cheap" isn't possible.
First look at range. Light aircraft that have a 2-ton+ payload have limited range, look at the upcoming Cessna 408, it can carry 3 tons but only has a 1,000 mile ferry range. In order to increase the range, you need to carry more fuel, reducing your load. So lets say you dedicated 1 ton to a fuel tank (neglecting the weight of the tank/piping), that gives you an extra ~300 gallons, maybe an extra 500 miles (I don't have info on fuel burn rates at this time, just a generous guess).
Now, you want 8,000 miles, so you need something that can carry a lot of fuel. Now you're in the 767-ish sized territory, so payload won't be a problem, nor will range (when you add , but it will be expensive. A new 767 cost around \$100 million, or you can lease it for between \$600,000 and \$650,000 per month.
If you need an aircraft with air-drop capability, now you need to find something ex-military. A C-130 won't do (range is only about 2000 miles), even the gigantic C-5 Galaxy only has a 4,800 mile range. There just doesn't exist an aircraft to do this.
Add on top of that the cost of developing an autonomous flying system with take-off and landing capability, you are well into the $200 million range at this point.
So now what? You've got a 767 with large payload capability (but no air-drop) that can fly somewhere and back. Time to fly right? Nope! You now (and have been for some time) need to deal with regulatory requirements for multiple jurisdictions since you are crossing a lot of unrelated airspaces in your flight. You need to coordinate demonstration flights and paperwork, along with all the compliance needed for a "one-of-a-kind" aircraft. Many won't want you flying over populated areas, or may deny you completely. Might have to do some lobbying ($$$).
Ok... got it, regulations approved, flight approved, aircraft bought, cargo bought, time to fly? Almost, try getting insurance. It will be expensive, probably hundreds of thousands per month.
A flight crew is the least expensive thing here
If you are serious about this, save yourself \$100m to \$150m and hire a qualified crew. You don't have to go through the development of this automated system, the regulatory approvals, the test flights, the insurance costs, the reluctance to have a 90+ ton aircraft flying without any operator intervention at 500 miles per hour and just hire somebody to fly it.
Pilots aren't the most well-paid, but lets say you have 2 crews making \$100,000/year each ($400,000/year total) with benefits. That's \$33,333/month, for the cost of development (lets say \$100m) of the automation system, you can employ these people for 250 years.

Hey now, a C-130 or C-5 can fly non-stop 'round the world! Of course, they need a bit of tankering on the way and that only increases the cost... ;)

@FreeMan Sure, just add another few hundred million for tanker drones. I'm sure drone-to-drone aerial refueling will make your insurance agent's day.

@StephenS there was a mention of cost and a wink there ;) indicating that this was sarcastic. Unless the long weekend has decalibrated my sarcasm detector...

@FreeMan Sorry, I thought my second sentence made the sarcasm clear.

tap, tap, tap... Yup, sarcasm detector is broken. Sorry, @StephenS, I'll get this thing in for service right away!

As others have pointed out, the range you're thinking of is unrealistic.
There are other problems too: making sure your dropped load doesn't hit anyone and will land in the right spot (and not, for instance in the nearby swamp) is not easy.
And in regions remote enough to qualify for this service, you can't count on qualified personnel to be available to receive and use medical supplies.
Have you looked at the way this is being handled today? Organizations like Mission Aviation Fellowship use general aviation aircraft like the Quest Kodiak and Cessna Caravan to fly personnel and supplies to remote locations.

There is no shortage of air freight companies that can quickly ship cargo to nearly any airport in the world, and since they have economy of scale and lots of experience dealing with regulators, you will never beat their costs.
Africa's main logistics problem is the so-called "last mile": getting supplies from an airport to where they're actually needed. In most cases, the range is only a few dozen to a few hundred miles (which could take days by truck, assuming you don't get robbed or killed) and the weight for any individual shipment will be fairly small as well. These trips could be entirely domestic, which makes them much easier from a regulatory standpoint. And it's only when flying such short, light trips that pilots become a non-trivial part of the cost.
So, to best solve this problem, you need a fleet of small and much less expensive drones, with a few at each major airport in each country, plus some staff to load and refuel or recharge them. And that is a much easier mission than what you proposed.

Not to drag this OT and into the political realm, but, unfortunately, it's the "last mile" that often doesn't happen because of corrupt governments (from local to national).

@FreeMan, coming up with a technical solution is certainly the easy part here. The political and organizational side is the real issue.

There is even an example to follow: https://youtu.be/jEbRVNxL44c

The best aircraft for your job is a used Boeing 767.
As others have pointed out, a UAV the likes of a MQ-9 or RQ-4 costs at least $200M and comes with a small payload and regulatory problems as well. Fortunately, there's a much better and cheaper option. In a 2015 article, Business Jet Traveller suggested a used Boeing 767 as a cheaper alternative to large business jets.

For well-heeled individuals (see below) who worry less about fuel costs, though, the 767 can offer a lot of aircraft for a relatively low price. [...] You can pick one up in airline configuration for as little as \$9 million, and that’s a pretty good deal for a big—and I do mean big—airplane that can fly 8,000 miles nonstop at a top speed of 469 knots. (A new 767 with an airline interior will set you back $185 million.)

They included a detailed cost breakdown, which can be summarized as follows:
Direct operating cost           $14k/h
Annual fixed operating cost     $1M
Annual deprecation              $1M
Total cost at 500h/year         $20k/h

If you buy a used 767 for \$9M, sell it a year later for \$8M, and in the meantime you fly a 10h mission per week where you drop 10 tons of cargo, you will end up with a total cost of \$10M for that year or \$20k per ton of cargo.
The 767 has a much larger cargo capacity, but you have to drop it somehow. I assumed you will carry a 50kg bag on each passenger seat and the crew drops these bags out of the door manually. You have to charge \$1000 per bag to break even. If you fly shorter missions out of port cities in Africa (Cape town, Lagos, Mombasa) you can reduce the cost per bag to \$200-500.

Your best bet might be the Aurora ORION UAV. It's essentially a twin engine plane turned motor glider. It offers a payload of up to a ton with a range of up to ~5,000 km. Or a ferry range (no cargo, just fuel) of 24,000 km. Being a glider design, its long distance cruise speed is below 150 km/h. So a long distance flights will take many days.

What you're looking for is essentially to combine several goals each require more fuel: Distance and load. With a given fuel capacity, stretching either will reduce the other. And both targets (8000mi, 2 t) are each for its own already hard to reach. While business jets like Embraers 300 Series or Pilatus PC-24 come close in carrying capacity, their range is way below 2000 miles. So if at all, it'll take a (small) commercial airliner like an ATR-42 in a special, maximum distance configuration. That's a 50 seat plane with a price tag of about 20,000,000 USD - just the plane. Any drone with the specs you want won't be any less.

So in short, long range, light-ish payload, speed doesn't matter and as cheap as possible.

1-2 tons ist not anywhere 'light-ish' in aeronautics. And for sure not cheap. Especially not when asking or record distance/duration.

Additional points that come up would be
Pressurisation - While you may think you can go without, your medical supplies can't, they are packaged for a certain air pressure range. Not only do you want the pills to be plopped out from their blisters, blood spilled from their tubes and vaccines floating in their outer containers might be equally undesired. Pressurisation is more important for fright than pilots. Humans can always go for personal gear providing oxygen.
And no, flying low isn't an alternative, as drag at low altitude will cut maximum distance a lot. There is a reason jets scratch the ozone layer in every flight - less air, less drag. Without going up there modern long distance flight would be as well impossible.
Flight Path - Sure, on the map it's easy to cross from Alexandria all the way to some remote town between Bangui and Kisangane. In reality each flight will be more paperwork than filing taxes for a multinational corporation. Each country on the way wants to have their say - and some even change their mind after everything has been signed. Keep in mind, each and every one of them has the means to simply pull your drone off the air.

Long story short, if this is really ment to work, go for a reasonable concept of medium distance (<1000km) flights with adequate load and managable handling.

I remember reading about how much trouble it was for those people trying to get permission to fly a balloon across various countries in their around the world attempts.  Trying to get the permissions for a UAV of sufficient size to do something like this would be even more difficult.
Assuming the various flight permissions from the various countries could be acquired, you might want to consider an airship (e.g. Goodyear Blimp, Zeppelin, etc) instead.  Speed is probably not critical here, so a slow moving airship would probably be acceptable.  You would still be better served with a manned crew instead of trying to make it autonomous.
On the other hand, doesn't UPS and FedEx deliver everywhere? :)


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How long can a turbine-engined aircraft be left fuelled?
Presumably, if one is going to be keeping an aircraft on the ground for a long time without running the engines or the APU, the aircraft's fuel tanks will need to be drained and purged...

...to prevent potential damage upon return to service due to degradation of the fuel over time...1
...to forestall a potential explosion hazard from fuel vapor in the fuel system and coming out of the tank vents on hot days...
...and to protect the local environment, both from the aforementioned venting of fuel vapors and from liquid fuel potentially draining out onto the ground should a leak develop while in storage.

However, on the flip side, as @Bianfable's comment points out, defuelling an aircraft has a significant disadvantage of its own; empty fuel tanks are vulnerable to condensation, which can cause accelerated corrosion and (in colder climates) ice accumulation.
Piston-engined avgas-burners generally have to be defuelled if they're going to be sitting inactive for six months or more; how long, generally, can a turbine-engined aircraft be left on the ground with jet fuel in its tanks?

1: Removing the fuel at the end of the storage period would also work for this purpose, but that would a) waste a planeload of fuel and b) involve draining the fuel system of various tarry fuel-degradation byproducts rather than just ordinary fuel.

Related: What do you need to do to bring a 737 Max back in service after 6 months in storage?

FWIW, the downside of empty tanks is condensation.

Turbine-engined aircraft use jet fuel, which is more similar to diesel than gasoline in handling.
It does not produce an explosion hazard, unless heated up in an enclosed space with a lot of headroom. Environmental damage is a concern, but generally a minor one. The fuel is not volatile and remains usable, but might partially degrade from oxidation.
A DTIC study has indicated that for the typical conservative 2-year storage period, jet fuel is stable even without additives. However, an antioxidant is recommended to prevent degradation for 3 years and beyond.
For longer storage terms, some degradation is observed, but generally a 10-year period can be considered safe for properly treated fuel. This is temperature-dependent, in hot climates, the safe storage period is reduced.
Given that corrosion can occur in empty tanks, the more practical solution is to add antioxidant for prolonged fuel stability, rather than defuel an aircraft.
For very long-term storage with a return to service later, the preservation procedure includes a lot of steps. In most cases, fuel is expected to stay in the aircraft. If draining the fuel, usually when return to service is uncertain, the aircraft's parts are treated with preservatives to allow for later restoration or cannibalization.

Technically, it's kerosene, not diesel.  Regardless, it's a lot more stable than I'd thought.  What about aircraft whose fuel tanks contain wide-cut fuels (gasoline-kerosene mixes, generally with the former predominating) like Jet B, though?

@Vikki I'm not sure. Jet-B is mostly used in Canada and Alaska. Europe and Asia use Jet-A-1 and TS-1, for both cold and normal weather, which are similar to Jet-A but lighter, and they store as well. Wide-cut fuels might call for a distinct question.

Distinct question asked.